[plt-scheme] HTTP or Cookie client side code

FWIW, I'm moving Httper and my new URI library off the backburner, so a
new Httper should be out very soon with UriFrame totally gone.

I plan to add client cookie management features shortly thereafter.  (I
need it myself immediately for a Web-scraping project.)  Other feature
requests welcome.

API-wise, Http will quickly be moving more towards composable
procedures, and the exposure of "class.ss" to represent an ADT will be
replaced with normal procedures and opaque objects.

BTW, one Httper application recently passed the milestone of 1E^9 HTTP
requests.

Oh, if a commercial entity needs an efficient HTTP library that works
with *multiple* Scheme implementations, I've been playing with how to
implement this well (using another hairy protocol as a learning
project).  It's substantial work that I can't justify right now, but
funding could make this glorious dream a reality.